This invention relates to a magnetron, and more particularly to a sealing structure for preventing leakage of electric wave.
Since a magnetron can generate microwaves at high efficiencies it is widely used in radar apparatus and microwave ovens. However, if the connection between a magnetron and such apparatus is not perfect microwaves would leak to the outside thereby decreasing the effective output of the magnetron. Moreover, such leaked wave causes radio frequency interference and emission effect of nearby electric machines and apparatus. If electrical connection is not perfect electric spark occurs.
FIG. 1 of the accompanying drawing shows one example of an inner magnet type magnetron widely used in microwave ovens comprising a yoke 11, a cylindrical anode electrode 12 and a cathode electrode 13 at the center of the anode electrode. A plurality of vanes 14 are secured to the inner surface of the anode electrode to extend toward the cathode electrode. Permanent magnets 15 and 16 are secured to the opposite ends of the anode electrode 12. The magnets are provided with conical pole pieces 17 and 18 respectively for passing magnetic flux through an interaction space defined by the cathode electrode 13 and the vanes 14. Although not shown in the drawing, a stem structure is secured to the pole pieces for supporting the cathode structure 13. The pole piece 17 is sealed to the lower end of a cup shaped metal cylinder 19 extending through the central opening of the annular magnet 15. The metal cylinder 19 projects through the yoke 11 to support a cylindrical insulator 20 which in turn supports an evacuation pipe 21 made of copper, for example, which function as an antenna terminal. One end of an output antenna lead wire 23 connected to one vane 14 is supported by the evacuation pipe 21. The evacuation pipe 21 is covered by a protective metal cap 24 which also functions as an antenna terminal. The metal cylinder 19 functions not only to form a terminal coupled to the antenna terminal but also to shield electric wave radiated from the antenna terminal. An electric discharging tube comprises the anode electrode 12 having a plurality of vanes 14, the cathode electrode 13 with stem structure for supporting it, pole pieces 17 and 18, the metal cylinder 19, the cylindrical insulator 20, the antenna terminal 21 or 24 and the antenna lead wire 23. Therefore, a portion comprised by the metal cylinder 19, the cylindrical insulator 20 and the antenna terminal 21 or 24 constitute an electric wave from the electric discharging tube. A metal cushion in the form of a metal washer 25 is interposed between the magnet 15 and its pole piece 17. Similarly, a metal cushion in the form of a metal washer 26 is interposed between magnet 16 and the yoke 11, and a gasket 27 is interposed between the yoke 11 and the metal cylinder 19. A plurality of cooling fins 28 are secured to the outer periphery of the anode electrode 12.
FIG. 2 is a partial enlarged sectional view showing a manner of preventing the leakage of the microwave energy when the magnetron shown in FIG. 1 is combined with microwave apparatus. In FIG. 2, an annular projection 31 of the wall 30 of a microwave oven or a waveguide is abutted against the gasket 27 made of a metal wire net, for example. Thus, the gasket 27 is clamped between projection 31 and metal washer 26 thereby preventing the microwave energy radiated from the antenna terminal from leaking to the outside. Accordingly, the microwave energy is efficiently transmitted to the waveguide of microwave oven.
However, with the construction described above, although the gasket 27 is strongly urged against the metal washer 26 by the projection 31, when the gasket 27 is heavily deformed by the projection 31 the thickness of the gasket would become non-uniform or shrink. Consequently, a gap is formed between the gasket and the metal cylinder 19 thus breaking the electrical connection therebetween. For the purpose of assuring easy fit between the metal cylinder 19 and the metal washer 26, the inner diameter of the metal washer is generally made to be slightly larger than the outer diameter of the metal cylinder with the result that a gap is formed between the gasket 27 and the metal cylinder 19, a portion of the microwave energy would leak to the outside through such gap. Further, when the electrical connection between the gasket 27 and metal cylinder is not good, an electric spark will strike therebetween in addition to the problem of leakage described above.